The present invention relates to a system for position determination of mobile objects, in particular vehicles.
It is known to use the GPS satellite navigation system (Global Positioning System) for positioning mobile objects, in particular vehicles. For this purpose a satellite receiving unit (GPS receiver) is installed on the mobile object (vehicle) which receives satellite signals for determination of their position coordinate (geographic width and length as well as the height over normal zero or kartesian coordinates X, Y, Z) from several not geostationary GPS navigation satellites which rotate on their orbits. The navigation satellites surround the earth on six orbits, and four satellites are located on each orbit. The conventional GPS receivers are as a rule designed so that the signals from four different navigation satellites can be received. The satellites send substantially their identification codes, a height-accurate, synchronized time, and their corresponding position. The GPS receivers calculate the distance to corresponding GP satellites, by measuring the time (running time measurements), over which the signals travel the from satellite to the receiver. Thereby with the suitable evaluation process, three-dimensional position determinations are performed. With such a system, however, only an accuracy of approximately, xc2x1100 m is obtained. The inaccuracy has various causes. One reason is the artificial worsening of the GPS satellite signals for many users, in contrast to authorized military users. A further reasons are orbit errors of the navigation satellites as well as inaccuracies of the satellite clocks. During passage of the satellite signal, its way to the GPS receiver through the ionosphere and troposphere further inaccuracies occur because of the alternating action with the propagation medium. Moreover, the constellation of the satellites in xe2x80x9cvisible fieldxe2x80x9d of the GPS receiver influences the accuracy. An accuracy +/xe2x88x92100 m is however not sufficient for many applications.
For these reasons a standard navigation system for automobile is reinforced, in addition to the GPS receiver, during the travel with various sensors arranged on the vehicle, such as for example wheels sensors, a speed measuring device, direction sensors/steering angle sensors, which produce data in connection with digitalized street maps. In this way, an accuracy which is sufficient for a travel route guidance for the position determination is obtained. This system however presumes that the mobile object moves only on known streets which are stored with digitalized street card. The higher accuracy requires thereby a limitation to the xe2x80x9cmovement freedomxe2x80x9d provided by the system. The system is not suitable to be used for position determination of vehicles for example in the agriculture, since a field does not have characteristic features required for the above described system, such as laterally limited streets with defined widths, indicated holding points such as intersections, road overpasses, curves with small curve radius.
Another alternative for the increase of the accuracy of the GPS satellite navigation system are so-called differential GPS system (DGPS). Here a stationary reference GPS receiver with exactly known position coordinates determines correction data for the above described errors (for example orbit errors, time error). These correction data in form of a correction signal are transmitted to the mobile GPS receiver for example by radio. From this correction signal, the mobile GPS receiver calculates corrected position data. Such a system is disclosed for example in the German patent document DE 41 36 136. In this reference, it is described how the radio sensor is used for sending the correction signals. The presumption for the efficient use of such a DGPS System is that the mobile GPS receiver receives three or four corresponding satellites, which also receives the reference GPS receiver and makes available for the correction data. Moreover, the error for the mobile GPS receiver and the reference receiver must be the same. This however is not the situation when the reference GPS receiver and the mobile GPS receiver are assembled very close. With the great distances between reference GPS receiver and the mobile GPS receiver, these presumptions are no longer valid, since then especially the atmospherically caused error (influence of the ionosphere, troposphere) are different influences, since the satellite signals cover different paths. With such a DGPS System, the accuracy of approximately xc2x15 m can be obtained. However, with the required radio connection a further disturbance source is provided in the positioning system. In accordance with several measurements, errors of up to 50% of the operational time are possible.
Accordingly, it is an object of the present invention to provide a system for position determination of mobile objects, which avoids the disadvantages of the prior art.
More particularly, it is an object of present invention to provide a system for position determination of mobile object based on a satellite navigation system, which is improved in simple and cost-efficient way as to the accuracy and reliability, and also to provide a method of improvement of the position determination.
In keeping with these objects and with others which will become apparent hereinafter, one feature of present invention resides, briefly stated, in a system for position determination of mobile objects, in which correction signals are received from two or more stationary reference satellite receiving stations and supplied to an electronic evaluating and processing unit. In the evaluating and processing unit from the correction signals, different reference satellite receiving stations determine with an evaluating algorithm an improved error correction signal. The thusly determined error correction signal is then transferred as a correction signal to the satellite receiving unit arranged on a moveable object for a position correction.
The evaluation algorithm contains quality criteria, with which the quality of different selection signals is continuously evaluated. It is therefore provided in a design of the inventive system to compare the different correction signals with one another and to determine the best correction signal on the satellite receiving unit arranged on the mobile object for position correction.
In accordance with a further feature of the inventive system, it is provided that the different correction signals are combined with one another. This can be performed for example by an average value formation. Moreover, it is provided for example to remove the correction data for the orbit error of the satellite from the correction signal 1 and the correction data for the influence of the ionosphere from the correction signal 2.
With the inventive system the accuracy and reliability of the position determination of mobile objects is substantially improved when compared with the simple differential GPS system which evaluates only a correction signal. The obtainable accuracy is within the region of xc2x11 m. Thereby the applications can include such as for example the position differentiated and controlled regulation of working organs in the agriculture (application of fertilization and plant protective substances, product mapping, etc.). In addition to the accuracy, also the reliability of the system is increased since no longer the dependency from a single, eventually not permanently available reference station is provided.
The inventive system is not limited to the position determination of vehicles. For example, it can be used for mobile GPS receiver which are carried by operators for positioning purposes.
The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.